In the manufacture of saw blades, for example, tungsten carbide tips are fixed by any suitable means (such as brazing) around the periphery of the blade to form the teeth thereon. While the tips will be supplied to the blade manufacturer with the appropriate shape within reasonable tolerances for the cutting operations they are eventually to perform, a final grinding operation is always required on them to ensure they are sharp and have the more accurate tolerances required in the final product.
The final grinding operation may be conducted in one or two stages depending on the particular saw blade being made and this is achieved in a grinding apparatus comprising means to hold the blade in a number of different angular positions to bring each tip in turn into proximity with one or more grinding wheels disposed in the apparatus and adapted to be moved across each tip to grind it in a particular plane. The grinding wheel is rotated by the apparatus to perform the grinding operation and comprises a hard composite material containing industrial diamond particles capable of grinding and removing material from the tungsten carbide tip.
In the process, the feed rates and relative speeds involved between the tip and the grinding wheel generate considerable frictional heat, not to mention very fine tungsten carbide dust. Thus a coolant is arranged to wash each tip as it is ground and the coolant comprises water and various known additives to assist the grinding process. Not least of these additives is a rust inhibitor to nullify the corrosive properties of water.
Environmental considerations alone would discourage disposal of the coolant after use, but in any event the cost of the various additives employed would prohibit employing fresh coolant on a continuous basis. Thus each grinding apparatus further comprises a coolant sump into which used coolant drains and from which a pump collects the stored coolant for recycling to the grinding region of the apparatus.
The tungsten carbide dust generated during the grinding process is of course largely entrained and held in suspension in the coolant and because it is so fine it has no opportunity to settle out in any quantity in the sump before it is recycled with the coolant. Thus after only a matter of hours after a fresh charge of coolant is introduced to the grinding apparatus, the coolant becomes heavily contaminated with primarily tungsten carbide dust. Of course, dust is also generated from wear of the grinding wheel, but this accounts for less than 10% of the total dust generated.
This contamination leads to further problems. One problem is an environmental one in that some of the coolant at the point of grinding gets converted into a spray which enters and carries into the atmosphere the entrained dust. Thus in some jurisdictions, it is a requirement in such grinding apparatus to provide a screen and vacuum extraction apparatus in the grinding region to remove contaminated air from the region and away from the potential surroundings of the grinding apparatus operators. The need for this would be considerably reduced if clean coolant was employed all the time. A further problem is simply the contamination of the apparatus, and particularly its moving parts, with a fine, very hard dust which significantly reduces the working life of the apparatus.
It has, of course, long been appreciated the benefits which would accrue from employing clean solution, and an obvious answer is to filter the coolant before it is recycled. However the average size of particles suspended in the coolant is of the order of one micron and this necessitates the use of disposable ceramic filters and a pressure system to force the coolant through the filter. Such an arrangement is prohibitively expensive.
Another solution is to provide a large central coolant reservoir serving a multitude of grinding machines and into which the sump of each machine drains and from which fresh solution is drawn. The hope is that, given sufficient time and sufficiently still conditions, at least the larger heavier particles will have settled out of suspension before the coolant is reemployed. However, it is the small light particles which are particularly responsible for the problems outlined above, because it is they which get carried into the vapour spray and which tend to drift towards operators and deposit themselves in unwanted places. It is unlikely that the central reservoir can be rendered sufficiently large, given economic constraints, to ensure settling out of these small particles. Besides which, smaller particles may be held as a colloid rather than in suspension and in which case will never settle out.